You will know this tune already: that composers have forgotten how to write for singers and they no longer care about text. The voice has become just another instrument, words are simply screeched syllables, their meaning incidental. No wonder the top singers steer clear of contemporary music: it isn't what their voices are for, nor is it what their fans want to hear.

Well, it's true that plenty of recent vocal music sounds unnatural, inexpressive. Who could blame singers for not wanting to perform it? Yet you might reverse the formula: while the finest singers are reluctant to make themselves available, composers are less likely to make the best use of vocal resources. Thank goodness, then, for a singer like the American Dawn Upshaw, in demand at opera houses and concert halls throughout the world but still committed to new music. Her recording of Henryk Gorecki's Symphony No 3 has sold a million copies, and next week at the Proms she gives the European premiere of Judith Weir's Natural History, a piece written specially for her.

For many singers of Ms Upshaw's calibre, song is little more than something to be accommodated on the way to performing opera. It is much more than that to Ms Upshaw, for whom contemporary song matters just as much as established repertoire: "Song has always been my first love. When I first decided to go for a career in music, song and chamber music were all that really interested me. It wasn't until I got to New York that I realised that I probably needed to get involved in opera. I had teachers who were working in contemporary music, so that came along quite early, and I've always found it exciting to come across new ideas. I feel that working with a clean slate brings out a different side of me. I enjoy actually working with composers, trying to get into their heads, to figure out why they've made the choices they've made. It's important for the life of the music that at least some of us get involved with new music, otherwise we're not going to find the pieces that will live."

Ms Upshaw acknowledges that there was a period when many composers ignored the singing qualities of the human voice: "I have worked on some of the repertoire written in the 1960s and "70s, and a lot of it I find ungratifying to sing. Sometimes I think right now that we've maybe gone a little too far in the other direction. I get sent a lot of music, but I don't hear a lot that is speaking with a really individual voice, an individual style. Much of it has been tonal, and of course there's nothing wrong with that, I love tonal music; but it seems to be recalling an earlier time without having anything new to say in terms either of expressing the text, or of using tonality. When that's the case, I get bored, I lose interest."

In these timid times, it is a rare singer who'll say anything against a return to tonality, although Ms Upshaw is not an unequivocal admirer of Schoenberg, the bugbear of those who blame all music's problems on the abandonment of tonality. "I recently sang Schoenberg's Pierrot Lunaire for the first time, which was an incredibly powerful experience. My reactions to the piece were strong, it was a wonderful exercise in terms of stretching my abilities, learning what I was capable of.

"To tell you the truth I don't really know what I thought, and I couldn't tell you for sure that I loved it, but there is something fierce going on that is truly inspired, perhaps genius, and I'm glad that I'll be doing it again in February. While I don't think Schoenberg would appear on my list of favourite 20th-century composers, I don't think blaming him for everything that has happened since makes sense either."

For Ms Upshaw, singing is not just about notes on the page. She has to feel touched by the text, even if it's not in English. Some composers have gone so far as to ask her to suggest the text that they should set, but, she says, "I've never directed a composer in that way. My feeling is that, if it doesn't begin with the composer themselves, then it won't be as inspired. Usually they'll send me things when they're just getting started in the composition, and if I feel no connection with the text, I'll try to be honest and suggest that they find someone else, which is tricky. Then there have been occasions when my response to the text has been completely different from the composer's; let's say I take something extremely seriously, but they set it in a rather frivolous manner. Then I'm stuck. Those kind of experiences are not fun."

No such problems with Judith Weir's Natural History, a work rapturously received when Ms Upshaw gave its first performance in Boston last January. The piece is a setting of ancient Taoist texts, each a compact scene depicting man's relationship with nature. They require a singer with a beautiful voice, but also with the ability to tell a story, characteristics Ms Upshaw possesses in abundance. "Judith sent me the texts before she wrote a note, and I liked them very much," she recalls. "She clearly felt strongly about them. When it came to the music, I was a little concerned before the first rehearsal, because it doesn't sit in a part of my register that I can count on sailing out over an orchestra, but it turns out that Judith has orchestrated it in such a way that there is no problem. The simplicity of the text comes through in the simplicity of the writing of the piece, and I think part of that is because she is not keeping me at the extremes of the upper and lower register so much."

Ms Weir, then, knows how to get the best out of the voice and Ms Upshaw is pleased to offer her best in return. Plenty of other composers are keen to work with this cultured performer, whether in opera or in song: over the next two years, she is scheduled to sing in new operas by Esa- Pekka Salonen, Kaija Saariaho and John Harbison; and tomorrow she'll be singing in the Salzburg Festival world premiere of Philip Glass's Symphony No 5. For Ms Upshaw at least, it's not all the same old song.